urbaf2

146 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

In this diagram the heat of interaction between the anhydrous acids, H 2 SO 4 ,

and HNO 3 , calculated on 100 g of mixture, has been presented, where Q is the

heat of mixing (in calories) and x is the H 2 SO 4 content in the mixtures (wt. %).

The heat maximum corresponds to x = ca. 67% of H 2 SO 4.

Gelfman points out that the heat of interaction between sulphuric and nitric

acids is the heat of reaction for:

HNO 3 + H 2 SO 4 <-> NO 2 + + HSO 4 - + H 2 O (2)

The interaction heat must not be identified with the heat of mixing. On adding

water to The mixture, the equilibrium is shifted to the left. Therefore the heat of

1600

1400

1200

1000

800

600

400

200

0 10 20 30 40 50 60 70 80 90 100

x. %

FIG. 20. Dependence of the heat of mixing Q of anhydrous HNO 3

and H 2 SO 4 on the content of H 2 SO 4 (x %) in the mixture (Gelfman [5]).

interaction between sulphuric and nitric acids in the presence of water is lower

than that between anhydrous acids.

During nitration partial separation of the acids takes place. This is the inverse

of the mixing process. The absolute value of the thermal effect of separation equals,

of course, that of mixing, but its value is negative. For this reason the thermal

effects of nitration calculated so far have been too high.

For example, the conversion of phenol into picric acid is accompanied by the

generation of 917.4 kcal per kg of the phenol, whereas the thermal effect, calculated

by earlier methods, was 1106 kcal/kg.

Similarly the heat of O-nitration of 1000 kg of glycerol with a nitrating mixture

comprising 5000 kg of HNO 3 (50%) and H 2 SO 4 (50%) is 251,669 kcal, though

according to earlier calculations it was 347,000 kcal.

ENTHALPY OF NITRATING MIXTURES

Figure 21 is a diagram by McKinley and Brown [6] showing the relative enthalpies

of nitric and sulphuric acids and their mixlures. Another diagram (Fig. 22) shows

the relationship between the specific heat of mixtures of acids and their composi-

tion.